Singapore

Despite being a bona-fide bulk insulator, diamond develops an intriguing two-dimensional (2D) p-type surface conductivity when its surface is terminated by hydrogen and exposed to appropriate surface adsorbate layer such as atmospheric water as a result of the surface transfer doping process. Consequently, the surface of diamond presents a versatile platform for exploiting some of the extraordinary physical and chemical properties of diamond, leading to applications such as chemical/biological sensing and the development of high-power and high-frequency field effect transistors (FETs).

In this talk, I will describe our recent work on the surface transfer doping of diamond by transition metal oxides (TMOs). Specifically, I will show that by interfacing diamond with MoO3 or V2O5 a 2D hole conducting layer with metallic transport behaviours arises on diamond. Magnetotransport studies at low temperature reveal phase coherent transport in the 2D channel with a transition from weak localisation to weak antilocalisation as temperature drops, and are analysed in the context of spin-orbit coupling induced by Rashba effect (Figure 1). The obtained spin-orbit interaction is a few folds higher than that in the air-doped channel or ionic liquid gated channel with similar hole densities, suggesting that the local carrier density could in fact be much higher than that obtained by Hall-effect measurements. We also demonstrate that this surface conducting channel can be exploited to build diamond surface electronic devices such as metal-oxide semiconductor FETs (MOSFETs). Lastly, the prospects for constructing novel quantum devices on diamond surface by making use of this highly tunable 2D conducting layer on diamond are also explored.

MATERIALS FOR HUMANITY (MH 21)

Engineering the 2D Hole Gas on Diamond for Carbon-Based Electronics

**Please view our Welcome to MH21 video.**

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Quasi-2D (Q2D) lead halide perovskites have emerged as promising materials for light-emitting diodes (LEDs) due to their tunable emission, slowed-down carrier diffusion, and improved stability. However, they are primarily fabricated through solution methods, which hinders its large-scale manufacture and practical applications. Physical-vapor-deposition (PVD) methods have well demonstrated the capability for reproducible, scalable, and layer-by-layer fabrication of high quality organic/inorganic thin films. Herein, for the first time, the full-evaporation fabrication of organic–inorganic hybrid ((BA)2Csn−1PbnBr3n+1) Q2D–3D PeLEDs is demonstrated. The morphology and crystal phase of the perovskite are controlled from 3D to 2D by modulating material composition, annealing temperature, and film thicknesses. The confinement of carriers in 3D layers and the energy funnel effect are discovered and discussed. Importantly, a record high external quantum efficiency (EQE) of 5.3% based on evaporation method is achieved. Moreover, a centimeter-scale PeLED (1.5 cm × 2 cm) is obtained. Furthermore, the T50 lifetime of the device with an initial brightness of 100 cd m−2 is found to be 90 min with a thin layer PMMA passivation, which is among the longest for all PVD processed PeLEDs. Overall, this work casts a solid stepping stone towards the fabrication of high-performance PeLEDs on a large-scale.

Scalable All-Evaporation Fabrication of Efficient Light-Emitting Diodes with Hybrid 2D–3D Perovskite Nanostructures

Upconversion is a nonlinear optical phenomenon that involves the emission of high‐energy photons by sequential absorption of two or more low‐energy excitation photons. Due to their excellent physiochemical properties such as deep penetration depth, little damage to samples, and high chemical stability, upconversion nanoparticles (UCNPs) are extensively applied in bioimaging, biosensing, theranostic, and photochemical reactions. This report will mainly introduce our recent research works of rare earth doped UCNPs for their applications in bioimaging, and cancer therapy.

Upconversion nanoparticles in luminescent detection, imaging, and therapy

Identifying new structurally and electrochemically stable fast ion-conducting solids so far is a slow trial and error process. To rationally accelerate materials design, a dependable rapid screening of materials is required so that promising structures can be shortlisted for higher level computational and experimental characterization. Here we report on our softBV bond-valence site energy-based automated pathway analysis that provides rapid simple visualization of ion transport pathways and yields meaningful approximate predictions of ion transport pathways from crystal structure models within seconds or minutes (a speed-up factor of 3 to 5 orders of magnitude compared to DFT-based NEB methods). Combined with a graphical user interface our software (available free for academic use from http://www.dmse.nus.edu.sg/asn/software.html) aims to enable experimentalists to quickly identify candidate solid electrolytes. Results are benchmarked against experimental and DFT NEB migration barriers. The approach now also comprises an AI-based dopant predictor focusing on bond-valence-based crystal chemical descriptors to assist experimentalists in exploring favourable substitutional doping strategies. 
We will also evaluate the predictability of absolute room temperature conductivities from static energy landscape analysis, bond-valence based empirical MD simulations and ab initio molecular dynamics (AIMD) simulations. While for small fast-ion conductor structures at high temperatures AIMD is the gold standard, the less reliable but computationally cheaper empirical approaches have an advantage in modelling complex disordered materials over longer periods, as they eliminate the need for risky extrapolations from high temperature AIMD simulations. Lastly we explore an extension of our bond-valence based approach by computationally efficient multibody terms in order to optimize their accuracy in studying complex interfaces in energy storage materials.

Tools for automated rapid screening of fast ionic conductors

In this project, two-dimensional (2D) materials are classified into the categories 2D magnetic materials and 2D non-magnetic materials. The 2D magnetic materials are further classified into two categories, namely 2D ferromagnetic materials and 2D anti-ferromagnetic materials. It is important to identify 2D ferromagnetic materials because they are used in various important applications such as data storage mechanisms and spintronic devices. Raw data from the 2D material databases such as 2DMatpedia was used in the project. Classical machine learning involving algorithms such as decision trees and support vector machines were used to classify the data obtained from the above databases into the respective categories anti-ferromagnetic, ferromagnetic and non-magnetic materials. Techniques such as deep learning and convolutional neural networks were also employed to classify the data. The best results for classification of the two-dimensional data into magnetic and non-magnetic materials were obtained for the classical machine learning algorithm, gradient boost algorithm with an accuracy of 93.37%. The most important features of the materials for determining the magnetic properties were band gap, number of magnetic atoms, maximum ionic character, presence of chromium and presence of manganese in that order. The magnetic materials classified were further classified into ferromagnetic and anti-ferromagnetic materials. The best accuracy for this process was obtained to be 94.44% using the classical machine learning algorithm, random forest classifier. The average ionic character, maximum average ionic character, yang-omega, and band-center were the key features for determining whether a material is ferromagnetic or anti-ferromagnetic. A web interface was deployed for the model and can be found here: https://twodferromagnetism-model.herokuapp.com. The code used for obtaining the results can be found here: https://github.com/RiyaBOT/ME4101A-ShethRiyaNimish.

Exploring two-dimensional ferromagnetic materials by machine learning

Many industries use deep learning algorithms to assist in the research and development of products. Therefore, we focus on using deep learning to help predict the parameters of new compounds and to help determine whether the compound is stable.
In this project, we explore the establishment of AI models through different algorithms for machine learning and deep learning. Then, we obtain the important features in the prediction process from the machine learning part, so as to provide a reference for actual research work. The accuracy of the model we get is analyzed and compared to determine the final algorithm and build the model. Using this model, we can predict the formation energy and energy above hull of compounds to determine whether they can exist stably. Finally, we integrate functions into a GUI, so that researchers can use this GUI to make stable predictions of any unknown new compounds easily and conveniently. Throughout the project, the accuracy of the model and the GUI have been continuously improved by us. And show these results in the following part.

Deep learning for accurate predictions of alloy stability

The setting up of the magnetic topological materials database provides the possibility of exploring in this field through the machine learning method. Here, we trained a support vector classification (SVC) model to predict the topological state of the magnetic materials. There are 551 of materials in the topological magnetic materials database, and for each material, the topological state is different for different U values as the date are the DFT results. So, different U values should be different data and finally, 1319 of data are used for the model training. Although the strongly imbalance of the data (only 87 of data are topological insulators (TIs)), the trained model still can reach high precision (0.99) for trivial materials, which means that almost all the un-trivial magnetic materials can be screened by our model. Then, the SVC model is used to predict the topological state of the magnetic materials in MAGNDATA database and select 65 materials as TIs and 15 materials as enforced semimetals (ESs) for all the four times prediction. Topological quantum chemistry method then used to test the topological state of the randomly selected 11 of the un-trivial materials and indicate that 5 of them are ES and 1 is TI. Then first principle method is applied to further inspect the topological state of the only TI. The atom unfilled orbitals and valence electrons are deemed as import features by our model, which is in consistence with the physical analysis results. This SVC model could help screen topological materials from large amount of magnetic materials.

Machine Learning Method to Fastly Screen Topological Magnetic Materials

Two-dimensional topological superconductivity candidate in a van der Waals layered material
Jing-Yang You ,1 Bo Gu ,2 Gang Su,2 and Yuan Ping Feng1,3,*
1Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551
2Kavli Institute for Theoretical Sciences, and CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China
3Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546

Two-dimensional (2D) topological superconductors are highly desired because they not only offer opportunities for exploring novel exotic quantum physics but also possess potential applications in quantum computation. However, there are few reports about 2D superconductors, let alone topological superconductors. Here, we find a 2D monolayer W2N3, which can be exfoliated from its real van der Waals bulk material with much lower exfoliation energy than MoS2, to be a topological metal with exotic topological states at different energy levels. Owing to the Van Hove singularities, the density of states near the Fermi level are high, making the monolayer a compensate metal. Moreover, the monolayer W2N3 is unveiled to be a superconductor with the superconducting transition temperature TC∼22 K and a superconducting gap of about 5 meV based on the anisotropic Migdal-Eliashberg formalism, arising from the strong electron-phonon coupling around the point, and the 2D superconductor is phonon mediated and fits the BCS mechanism with an Ising-type pairing. Because of the strong electron and lattice coupling, the monolayer displays a non-Fermi liquid behavior in its normal states at temperatures lower than 80 K, where the specific heat exhibits T3 behavior and the Wiedemann-Franz law is dramatically violated. Our findings not only provide a platform to study the emergent phenomena in 2D topological superconductors, but also open a door to discover more 2D high-temperature topological superconductors in van der Waals materials.

Reference:
[1] J-Y. You, B Gu, G. Su, Yuan Ping Feng, Physical Review B 103, 104503 (2021).

Two-dimensional topological superconductivity candidate in a van der Waals layered material

Two-dimensional (2D) Janus MoSSe have great potential for photocatalytic water splitting due to their novel properties induced by the unique out-of-plane asymmetric structures.1-3 Doping is an effective way to modulating the photocatalytic properties. The intrinsic defects are usually unavoidable when introducing extrinsic defects like transitional metal adatoms. However, to our knowledge, few researchers have investigated the synergistic effects on the photocatalytic properties when vacancies and adsorbed transition metals coexist on the MoSSe surface. Here, we systematically investigate the geometric, electronic, and optical properties of 2D Janus MoSSe with titanium doping and vacancies to explore their synergistic effects on photocatalytic activity. 
We find that there is effective attraction between the substituted or adsorbed Ti atoms and S/Se vacancies. The Ti adatoms dramatically extend the light absorption range to the infrared region. The S/Se vacancies coexisting with Ti adatoms will modulate the transition of photoexcited electrons, thereby enhancing sunlight absorption as shown in Figure 1. The Ti adatoms either existing alone or coexisting with vacancies introduce smaller lattice distortion compared with substituted Ti atoms, and these Ti adatoms induce smaller effective mass of charge carriers. The configuration of S vacancy coexisting with Ti adatoms on the Se surface exhibits the most significant synergistic effects and the best overall photocatalytic performance. 
Our work reveals the mechanisms and effects induced by doping and vacancies coexisting in 2D Janus MoSSe and also proposes a new practical strategy to improve the performance of 2D photocatalysts.


Effects of Vacancy and Ti Doping in 2D Janus MoSSe on Photocatalysis

We present a quantum-mechanical study of a B2-phase of Fe70Al30 alloy
with and without antiphase boundaries (APBs) with the {001} crystallographic
orientation of APB interfaces. Our motivation is to explain experimental
findings demonstrating a significantly higher magnetic flux density from 
A2-phase interlayers at the thermally-induced APBs in this material
and suggesting that the ferromagnetism is stabilized by the disorder in
the A2 phase. Our investigation of sharp APBs (without any A2-phase interlayer) 
indicates that they have moderate APB energies (~0.1 J/m2) and cannot explain the 
experimentally detected increase in the ferromagnetism because they 
often induce a ferro-to-ferrimagnetic transition. When studying
thermal APBs, we introduce a few atomic layers of A2 phase of Fe70Al30 into
the interface of sharp APBs. The averaged computed magnetic moment of Fe atoms
in the whole B2/A2 nanocomposite is then increased by 11.5% w.r.t. the B2 phase.
The A2 phase itself (treated separately as a bulk) has the total magnetic moment
even higher, by 17.5%, and this increase also applies if the A2 phase at APBs
is sufficiently thick (the experimental value is 2–3 nm). We link the changes
in the magnetism to the facts that (i) the Al atoms in the first nearest
neighbor (1NN) shell of Fe atoms nonlinearly reduce their magnetic moments
and (ii) there are on average less Al atoms in the 1NN shell of Fe atoms
in the A2 phase. These effects synergically combine with the influence
of APBs which provide local atomic configurations not existing in
an APB-free bulk. The identified mechanism of increasing the magnetic
properties by introducing APBs with disordered phases can be used as
a new designing principle when developing new magnetic materials.

Enhancing magnetism in Fe-Al alloys by increasing chemical disorder

Solid -state conductors have attracted wide attention due to their benefit of enhanced safety and capacity with application in batteries. Our computational tools--bond valence pathway analyzer (BVPA) have been reported to provide an automated pathway analysis method for large database of solid Ionics and a dopant predictor to easily monitor the site substitution of the lattice[1]. In this work, local coordination environment characterization and classification has been introduced to further understand the interstitial site and atomic sites in the crystal structure and to improve the predicted stability of modified lattice. The local coordination number evaluation is implemented based bond valence theory [2] with proofed credibility after benchmarked with most reported methods[3]. Over 35000 atomic sites have been analysed from over 7000 high quality structure obtained from ICSD database. Machine learning has been applied to divide the local environment into over 60 defined polyhedral environments. The polyhedral environments are further explored to understand the interstitial sites in ionic pathway.

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Engineering the 2D Hole Gas on Diamond for Carbon-Based Electronics

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